1. Field of the Invention
This invention relates generally to nuclear magnetic resonance imaging (commonly referred to as MRI). More specifically, this invention pertains to a method of creating a magnetic resonance susceptibility image of an object from it's MRI data using susceptibility weighted imaging and susceptibility mapping.
2. General Background Technology
Various magnetic resonance imagining techniques utilize susceptibility weighted imaging (SWI) to enhance the contrast between spatial discontinuities in the magnetic susceptibility within objects being scanned. SWI is described generally in U.S. Pat. No. 6,658,280, which is hereby and herein incorporated in its entirety by reference.
The ability to quantify local magnetic susceptibility is tantamount to being able to measure, the amount of iron in an object or body whether it is in the form of non-heme iron (such as ferritin or hemosiderin) or heme iron (de-oxyhemoglobin), or the amount of calcium or any other susceptibility affect causing materials including geometric effects. Recently, several methods of quantifying local magnetic susceptibility have been developed that utilize a fast Fourier transform approach. One of such methods utilizes the inverse of the Green's function.g(k)=⅓−kz2/(kx2+ky2+kz2)where kx, ky, and kz are the coordinates of the spatial-frequency domain of the phase data. That method, being derived from an ill-posed problem, is fraught with difficulties. The ill-posedness arises from zeros in the denominator of the analytic filter. To this end, various complicated and lengthy methods involving regularization, multiple scans acquired with the object being rotated between scans, and constrained least squares approaches have been practiced in an effort to improve results.
Whereas a least squares approach requires relatively accurate knowledge of the geometry of the object, methods using multiple orientation data require additional time for collecting data in different object orientations. The least squares approach in some cases also imposes a homogeneous-susceptibility constraint within an entire object. The direct inverse approach presented here overcomes these limitations as it primarily uses magnetic resonance phase information of the object from a single orientation and is a rapid and easily implemented technique.
Further, unlike SWI which has an orientation dependence on the phase behavior and hence in the end affects the final contrast, the present invention creates a data set that is independent of the object's orientation to the main magnetic field. Therefore, if one were to use the susceptibility data rather than the usual filtered phase data to create SWI data, the results will be superior in terms of not having an orientational dependence.